Embedded In-Cord NTC Limiter for PSU Inrush Current
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Solution Overview
Problem
High-capacity power supply unit (PSU) loads experience inrush currents exceeding 200 A, leading to relay failure and potential damage due to terminal blowout, and existing power distribution units (PDUs) lack active overcurrent protection, affecting AC source instrumentation stability and reliability under poor power quality conditions.
Innovation Solution
An embedded in-cord inrush current limiter device with a housing containing a switch, NTC thermistor, voltage and current sensors, and a microcontroller unit (MCU) that senses input voltage and inrush current, engaging the NTC thermistor to limit inrush current when thresholds are reached, and includes a reset button for arming or resetting the device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If high-capacity power supply unit loads are connected to PDU, then power delivery capability is improved, but inrush current exceeds relay rating causing terminal blowout and relay failure
Solution Approach 1:
The NTC thermistor is pre-installed in series with the power relay to provide inrush current limitation before the relay is exposed to high inrush currents from power supply unit loads. This preliminary protective measure prevents terminal blowout and relay failure while allowing the relay to maintain its rated power delivery capability.
Solution Approach 2:
The NTC thermistor acts as an intermediary component between the power source and the power relay. It temporarily absorbs and limits inrush current during power-on events, protecting the relay from excessive current stress while allowing normal operation once the thermistor cools down.
2Reliability
If active overcurrent protection devices are added to every rPDU outlet, then inrush current protection is improved, but device complexity and cost increase
Solution Approach 1:
The active overcurrent protection functionality is extracted from the complex rPDU outlet and implemented as a simple NTC thermistor component embedded in the power cord itself. This simplifies the overall device complexity while maintaining effective inrush current protection at each outlet.
Solution Approach 2:
The NTC thermistor provides self-regulating inrush current protection without requiring external control circuits or complex electronics. The thermistor automatically limits inrush current based on its temperature-dependent resistance characteristics, simplifying the protection mechanism.
3Device complexity
If inrush current limiting is implemented using passive NTC thermistor only, then device complexity is reduced, but protection effectiveness is insufficient under voltage dip conditions
Solution Approach 1:
The solution merges the passive NTC thermistor inrush current limiter with an active control system comprising voltage sensors, current sensors, and a microcontroller unit. This hybrid approach combines the simplicity of passive components with the intelligence of active control to provide comprehensive protection under various power quality conditions including voltage dips.
Solution Approach 2:
The microcontroller unit continuously monitors voltage and current waveforms through sensors and uses this feedback to intelligently control the power relay. When voltage dips or abnormal inrush currents are detected, the MCU adjusts relay operation to maintain protection effectiveness, overcoming the limitations of purely passive NTC thermistor protection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The device effectively limits inrush currents, preventing relay failure and protecting PDUs and end devices from damage, while maintaining AC source instrumentation stability and reliability under varying power conditions.
Implementation Method 1
a switch and negative temperature coefficient (NTC) thermistor serially connected to a first conductor
Data Source
Figure 1
Figure 2
Figure 3A
AI summary
An apparatus (118) and method for limiting inrush current to an end device provides an alternating current (AC) to the end device (110) via a power cord (106, fig.1) including first and second current carrying conductors. A voltage sensor (206) enclosed by a housing attached to a power cord and connected to the conductors senses an input voltage. A current sensor (204) within the housing senses an inrush current via the first conductor. A microcontroller unit (MCU) within the housing receives the sensed input voltage and inrush current. Based on the sensed input voltage, the MCU determines a voltage drop across a negative temperature coefficient , NTC, thermistor (210) serially connected, with a parallel switching device 208, to the first conductor. When the sensed inrush current and/or the sensed input voltage reaches a threshold level, the MCU engages the NTC thermistor (210) to limit inrush current by opening the switching device (208).